Various kinds of sensors or information-transmitting devices containing sensors (sometimes also called smart devices) continue to be installed in physical environments leading to a merger of the online and physical worlds. Mobile phones also contain several sensors that may provide data to applications installed on the mobile phone or to servers connected to the mobile phone via data networks. Software systems that create virtual environments are being instrumented to capture events in data sets (also called log files). Data sets of online and physical environments are being made available for computational purposes.